The present invention relates generally to automatic loop (“autoloop”) system for an electrical power distribution network and more particularly to an autoloop system that facilitates segmentation of the autoloop in the event of a fault.
Electrical utilities have a number of metrics that are used to track performance and customer satisfaction. These metrics, which include the system average interruption frequency index (“SAIFI”), the customer average interruption duration index (“CAIDI”), and for some utilities, the momentary average interruption frequency index (“MAIFI”). SAIFI measures the average number of interruptions that a customer would experience during a time period, such as a year. CAIDI measures the duration of the interruption that a customer would experience, and is generally a few hours per year. MAIFI measures the number of power interruptions that have a duration of less than five minutes that a customer would experience during a given time period.
Some or all of these metrics are also used by government regulators to aid in determining if the electrical utility is adhering to the regulations in maintaining a durable and reliable electrical distribution network. As a result, electrical utility and distribution companies have developed system architectures that minimize the duration and frequency of power outages. One technique uses a device called an “autorecloser” or simply “recloser”, in the protection scheme for the distribution system. A recloser is a type of circuit breaker that includes a mechanism that allows the circuit breaker to close or reconnect the electric circuit after an electrical fault is detected. The premise of the recloser is that many electrical faults on the overhead open wire system are transitory, due to issues such as a tree limb touching or falling on a power line for example. The recloser includes a controller that is programmed to make several attempts to reconnect before locking open the recloser. Generally, the feeder recloser is programmed with an operating profile that provides for one “fast” trip and one to three “slow” trips before finally locking open.
Another technique used to improve performance is to arrange the electrical circuits in what is commonly referred to as an autoloop system. In an autoloop, the distribution network is divided into two or more branch circuits that can be adaptively coupled in the event of a failure. Generally, a substation will provide two or more feeder branch circuits to a typical autoloop. A feeder recloser is positioned at the beginning of the circuit and provides the protection functionality discussed above. A second recloser is positioned at an intermediate position along the branch circuit. The second recloser may be a “midpoint” type recloser that opens in response to a down stream fault. In addition or in place of the midpoint recloser, the system may include a sectionalizing recloser that opens in response to a downstream fault or may open in response to a loss of voltage. Positioned at the end of the branch circuit is a third type of recloser known as a “tie” recloser. The tie recloser couples the branch circuit with a similarly configured adjacent branch circuit. A sectionalizing recloser is different from a tie recloser in that a sectionalizing recloser will open in response to a fault that occurs downstream under normal current flow condition. A sectionalizing recloser will also open after a predetermined amount of time following a loss of voltage.
The tie recloser is normally in an open position to prevent the flow of electrical current between the branch circuits. In the event of a fault between the feeder and midpoint that the feeder recloser cannot clear, the feeder recloser locks open and results in a loss of potential on the entire branch circuit. When this occurs, the midpoint recloser changes settings to trip/open at a lower current level and lock open after only one trip. The tie recloser is arranged to automatically close at short period of time, typically 15 to 100 seconds, upon the detection of the loss of voltage potential. When the tie recloser connects the two branch circuits, electrical current can flow into the branch circuit with the electrical fault. Depending on where the fault is located on the branch circuit, partial electrical service can be restored to the portion of the branch circuit that experienced the failure. For example, if the fault occurs between the feeder recloser and the midpoint recloser, when the tie recloser connects the circuits, electrical power flows into the branch circuit. Since the fault is still present, the midpoint recloser trips and locks open. However, electrical power is still available for the customers between the sectionalizer/midpoint recloser and the tie recloser. Thus the average duration of electrical power loss and the amount of affected customers are minimized.
It should be appreciated that the when the tie recloser closes to flow electrical power into the adjacent branch circuit, the electrical power will flow through the sectionalizing/midpoint recloser in the opposite direction from normal operation. As a result, the sectionalizing/midpoint recloser may react differently than desired to subsequent faults. Further, it is desirable to increase the amount of fault isolating devices on a circuit to reduce the amount of customers that are without power. However, the more devices that are included in a circuit, the more difficult it is to obtain a desired level of coordination between the devices. It has been found that on some circuits, the number of devices may not be increased without resorting to difficult and expensive communications schemes or extensive redesign of the circuit.
Thus, while existing electrical distribution systems are suitable for its intended purpose, there remains a need for improvements; particularly regarding the operation and coordination of devices in an autoloop system to increase the number of isolating devices on a given circuit to reduce the number of customers that lose electrical power during an outage.